Aliphatic polycarbonate is an easily biodegradable polymer and is useful for packaging or coating materials, etc. Processes for preparing polycarbonate from an epoxide compound and carbon dioxide is highly eco-friendly in that they use no harmful compound, phosgene, and adopt easily available and inexpensive carbon dioxide.
Since 1960's, many researchers have developed various types of catalysts to prepare polycarbonate from an epoxide compound and carbon dioxide. Recently, we have developed a catalyst for carrying out carbon dioxide/epoxide copolymerization. The catalyst includes a complex having an onium salt and a metal center with a Lewis acid group in one molecule. Use of the catalyst allows the growth point of the polymer chain to be positioned always in the vicinity of the metal in the polymerization medium for carrying out epoxide/carbon dioxide copolymerization, regardless of the concentration of the catalyst. In this manner, the catalyst shows high activity even under a high ratio of monomer/catalyst, exhibits high cost-efficiency by virtue of a decrease in catalyst need, and provides polycarbonate with a high molecular weight. Moreover, the catalyst realizes polymerization activity even at high temperature to increase the conversion, permits easy removal of the polymerization reaction heat, and thus is easily applicable to commercial processes [see, Korean Patent Application No. 10-2007-0043417 (May 4, 2007, Title: COORDINATION COMPLEXS CONTAINING TWO COMPONENTS IN A MOLECULE AND PROCESS OF PRODUCING POLYCARBONATE BY COPOLYMERIZATION OF CARBON DIOXIDE AND EPOXIDE USING THE SAME); International Patent Application No. PCT/KR2008/002453; Eun Kyung Noh, Sung Jae Na, Sujith S, Sang-Wook Kim, and Bun Yeoul Lee* J. Am. Chem. Soc. 2007, 129, 8082-8083 (2007 Jul. 4)]. Further, when the complex having an onium salt and a metal center with a Lewis acid group in one molecule is used as a catalyst for carbon dioxide/epoxide copolymerization, the catalyst is easily separated and reutilized from the copolymer after the polymerization. Thus, such a method for separately recovering the catalyst has been described in a patent application and a journal [Korean Patent Application No. 10-2008-0015454 (Feb. 20, 2008, Title: METHOD FOR RECOVERING CATALYST FROM PROCESS FOR PREPARING COPOLYMER); Bun Yeol Lee, Sujith S, Eun Kyung Noh, Jae Ki Min, “A PROCESS PRODUCING POLYCARBONATE AND A COORDINATION COMPLEXES USED THEREFOR” PCT/KR2008/002453 (2008 Apr. 30); Sujith S, Jae Ki Min, Jong Eon Seong, Sung Jea Na, and Bun Yeoul Lee* “A HIGHLY ACTIVE AND RECYCLABLE CATALYTIC SYSTEM FOR CO2/(PROPYLENE OXIDE) COPOLYMERIZATION” Angew. Chem. Int. Ed., 2008, 47, 7306-7309].
The complex of the above studies mainly includes Salen-cobalt compound ([H2Salen=N,N′-bis(3,5-dialkylsalicylidene)-1,2-cyclohexanediamine] (see the following chemical formula), obtained from a Schiff base ligand of a salicylaldehyde compound and a diamine compound. The complex is a tetradentate (or quadradendate) cobalt compound-based complex in which trivalent cobalt atom is coordinated with two nitrogen imine ligands and two phenolate ligands at the same time:

The complex may be referred to as a tetradentate (or quadradendate) Schiff base complex, and may be prepared according to the following reaction scheme:

The above tetradentate (or quadradendate) Schiff-base cobalt or chrome complex has been developed intensively as a carbon dioxide/epoxide copolymerization catalyst. (Cobalt-based catalyst: (a) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng, X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664. (b) Cohen, C. T. Thomas, C. M. Peretti, K. L. Lobkovsky, E. B. Coates, G. W. Dalton Trans. 2006, 237. (c) Paddock, R. L. Nguyen, S. T. Macromolecules 2005, 38, 6251. Chrome-based catalyst: (a) Darensbourg, D. J.; Phelps, A. L.; Gall, N. L.; Jia, L. Acc. Chem. Res. 2004, 37, 836. (b) Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005, 127, 14026.).